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 Report dateSiteEvent description
05000259/LER-2017-00121 February 2017Browns Ferry

On December 21, 2016, at 1228 Central Standard Time (CST), during a performance of the 4KV Shutdown Board (SDBD) C Undervoltage and Time Delay Relay Calibration and Functional Test, personnel discovered a detached restraining strap on a 4kV SDBD C Degraded Voltage Relay Timer. At 1835 CST, Operations personnel declared the relay inoperable. The timer retaining strap was replaced, and the relay was declared operable on December 22, 2016, at 1251 CST.

A Past Operability Evaluation determined that the timer was inoperable from October 5, 2016, until December 22, 2016, exceeding the Technical Specification allowed outage time.

The most likely cause of this event was human error. Rounding of the screw attaching the retaining strap to the backplane of the electrical cabinet in which the timer was housed allowed the retaining strap to become detached. This deficiency was not corrected despite testing and Quality Control verification. Corrective Actions include replacing the retaining strap using a longer screw, identifying relays previously installed under similar plant modifications, and ensuring that work packages for future installation of relays with seismic retaining straps contain steps to obtain adequate screw engagement during strap installation.

05000296/LER-2015-001, High Pressure Coolant Injection and Reactor Core Isolation Cooling Inoperable Due To No Suction Source Aligned13 April 2015Browns Ferry

On February 11, 2015, at 0820 Central Standard Time, Brown's Ferry Nuclear Plant (BFN), Unit 3, declared the High Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) systems inoperable due to no suction source aligned. During surveillance testing, the Condensate Storage Tank (CST) emergency discharge isolation valve energized and closed when the breaker was closed, isolating both systems from their suction source. It was subsequently determined that maintenance task. Operations personnel re-opened the isolation valve using the hand switch in the Control Room, restoring operability to the HPCI and RCIC systems.

The apparent cause of this event was inadequate design review of a 2010 plant modification which allowed latent design vulnerabilities to be introduced into the plant.

The corrective actions to reduce the probability of a similar event occurring in the future were to remove thermal overload heaters from the affected breakers, preventing valve closure when these breakers are closed; to review a sample of recent engineering change packages for quality of Design Review; to repair a faulty hand switch; and to implement a design change for the CST isolation valves for all three BFN units to prevent spurious operation of the isolation valve when the associated breaker is closed.

05000260/LER-2014-00123 December 2014Browns Ferry

On March 27, 2014, it was determined that the Browns Ferry Nuclear Plant (BFN) Required Actions of Technical Requirements Manual (TRM) 3.7.6, Electric Board Room Air Conditioning (AC) System, Condition B, would allow both BFN, Unit 2, Electric Board Room (EBR) AC subsystems to be inoperable for up to 7 days before declaring the Technical Specifications (TS) supported equipment in the EBRs inoperable. This allowance is contrary with the TS definition of "Operable-Operability" with respect to support systems. On two separate occasions in the past three years BFN, Unit 2, EBR AC System and its TS supported systems, were inoperable longer than allowed by TS. After further review of the condition, the causal analysis was revised and it was determined that BFN, Units 1 and 2, did not experience a Safety System Functional Failure (SSFF). This event is being reported in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(i)(B).

The root cause determined that BFN personnel failed to fully understand the difference between the Technical Requirements Manual (TRM) and Technical Specifications (TS) with respect to operability.

When the TS and TRM were implemented, BFN personnel failed to realize the intent of the TRM and believed it was at the same level of the TS.

The corrective actions to prevent recurrence include revising the TRM to clearly delineate the intent and use with respect to operability and revising the procedure for Technical Specifications, Licenses and Amendments to clearly delineate the role of the TRM and TS.

05000296/LER-2004-00224 January 2005Browns Ferry

On November 23, 2004, while Unit 3 was in steady state operation at 100% power, a main turbine trip and subsequent reactor scram occurred. All expected system responses occurred. A lightning strike occurred on the TVA 500-kV system approximately 40 miles distant from Browns Ferry. This strike resulted in a phase-to-ground fault on all three phases of the transmission line, and the electrical power transient caused speed perturbations on both the Unit 2 and Unit 3 main turbines. The rate of speed change seen on Unit 3 was slightly greater than the maximum rate anticipated by the turbine control system logic, and therefore the turbine speed feedback signals, while valid, were designated as invalid by the logic. With all turbine speed feedback signals designated as invalid, a main turbine trip on loss of speed feedback occurred in accordance with the system design, and a reactor scram occurred due to the turbine trip.

The event cause was that actual turbine speed changes exceeded those anticipated as possible by the turbine control logic, causing valid speed signals to be designated as invalid. Corrective actions included the adjustment of the affected logic settings, evaluation of the turbine speed response, and consideration of modifying the speed control and turbine trip logic.